> My claim was that sending electronic mail is a solved problem, because it is.
Agreed. I mean, I can _probably_ remember enough to be able to send mail using just telnet without even needing to look anything up. (And that mail, without any mime parts or urls - it very very likely to not fall into any of the spam filtering on my inbound email accounts...)
In the modern era, lots of mailservers run software that looks for weirdnesses in incoming email. I don't know, but I'd suspect that the typing delays of using telnet to send mail are likely to trigger their defenses.
SO I just tested. After working out that on macOS I now need to use nc -c instead of telnet... I _can_ send mail manually, but it _does_ end up in gmails's spam folder saying "Why is this message in spam? It is similar to messages that were identified as spam in the past."
Iain-2:~ iain$ nc -c gmail-smtp-in.l.google.com 25
220 mx.google.com ESMTP k5si5682618pls.209 - gsmtp
EHLO elided.org
250-mx.google.com at your service, [202.171.181.100]
250-SIZE 157286400
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
MAIL FROM: <bigiain@elided.org>
250 2.1.0 OK k5si5682618pls.209 - gsmtp
RCPT TO: <bigiain@gmail.com>
250 2.1.5 OK k5si5682618pls.209 - gsmtp
DATA
354 Go ahead k5si5682618pls.209 - gsmtp
Subject: Test
From: bigiain@elided.org
To: bigiain@gmail.com
Date: Fri, 31 Jan 2020 13:07:00 +1100
Foo baz nah
.
250 2.0.0 OK 1580436443 k5si5682618pls.209 - gsmtp
QUIT
221 2.0.0 closing connection k5si5682618pls.209 - gsmtp
Iain-2:~ iain$
Most mailservers don't send emails one character at a time. You appear to have a mental model of how a spam filter works that doesn't include fingerprinting the behavior of the sending server? Even open source mail servers do that, for example postscreen is a part of postfix.
Where it came from is one of the most important details!
Is this mail relay actually elided.org or is it impersonating elided.org?
There does not appear to be an SPF record for that domain so Gmail cannot determine if this is a fraudulent message, especially if the machine this request came from does not match the IP of the A record for that domain.
Agreed. I mean, I can _probably_ remember enough to be able to send mail using just telnet without even needing to look anything up. (And that mail, without any mime parts or urls - it very very likely to not fall into any of the spam filtering on my inbound email accounts...)